The present invention relates generally to electronic devices and more particularly to methods and devices for packaging semiconductor electronic devices, such as integrated circuit chips.
The packaging of integrated circuit chips for use in computers or similar devices typically involves the attachment of the circuit chips to intermediate device carriers which, in turn, are mounted to a printed circuit board, such as a mother board. The intermediate device carriers are generally considered necessary because of the large difference in the thermal coefficient of expansion between the integrated circuit chip and the printed circuit board. In a particularly well known type of configuration, the chip is mounted onto a ceramic module by xe2x80x9cflip-chipxe2x80x9d bonding wherein the I/O pads on the face of the chip are bonded to corresponding pads on the module by use of solder bumps or solder balls with solder reflow techniques. The opposite side of the ceramic module is provided with an array of pins or solder bumps which are positioned for insertion into a complementary array of holes or bump contacts on the printed circuit board. The printed circuit board typically includes a plurality of ceramic modules along with a number of interconnected lumped electrical elements, such as capacitors, transformers and resistors, that cannot be conveniently integrated into the chips or modules.
To insulate the integrated circuit chip from dust and other particles, a lid is typically bonded to the carrier device to form an enclosed cavity around the chip. With integrated circuits that are xe2x80x9cflip-chipxe2x80x9d bonded to the device carrier, heat dissipation paths are generally limited to the bump contacts on the face of the chip since the back of the chip is not in contact with the carrier device or the circuit board. As the contact bumps have become finer, their contact area has grown smaller, limiting the amount of heat dissipation through the bump contacts. To overcome this problem, the lid is typically fabricated from a thermally conducting material, such as metal, for thermally connecting the back side of the circuit chip to a heat sink, thereby allowing dissipation of the heat generated by the integrated circuit chip during electrical operations.
While the above-described technique for connection of integrated circuit chips to printed circuit boards is effective in many instances, it does have several drawbacks and limitations. One such drawback is that the lid and the carrier device typically have substantially different coefficients of expansion when heated. Thus, thermal cycling during electrical operations of the chip will tend to cause undesirable stresses or strains at the lid/module interface, which can lead to material or electrical failure. Material failure at this interface often results in delamination of the adhesive between the lid and module, which causes the lid to separate from the ceramic module.
One attempt to overcome this problem of dissimilar thermal expansion between the lid and the ceramic module is to use compliant adhesives with very low glass transition temperatures to connect the lid to the ceramic module. These compliant materials typically have low moduli and, therefore act as a shock absorber or cushion to absorb the stress resulting from the thermal expansion mismatch between the lid and the ceramic module. Although these compliant adhesives have reduced separation during temperature cycling, the adhesive typically deteriorates at elevated temperatures. Thus, for example, when the module is heated to high temperatures (i.e., on the order of 220xc2x0 C. to 250xc2x0 C.) to remove it from the printed circuit board, the compliant adhesive may degrade, allowing the lid to separate from the module.
What is needed, therefore, are improved devices and methods for packaging semiconductor electronic devices, such as integrated circuit chips. These devices and methods should be capable of forming a protective lid over the integrated circuit chip that will remain substantially adhered to the underlying substrate through relatively large temperature changes, such as during chip electrical operations or the component removal process.
The present invention is directed to a device and method for packaging an electronic device, such as an integrated circuit chip. As used herein, the term xe2x80x9celectronic devicexe2x80x9d includes, but is not limited to, a single semiconductor integrated circuit chip, a multichip module comprising two or more such chips located on and connected to an interconnection substrate, or a composite of one such chip or module that is flip-chip or otherwise bonded to either another such chip or another such module.
The electronic device package of the present invention includes an intermediate device carrier with a substantially planar upper surface and a plurality of electrical attachments for mounting an integrated circuit chip to the device carrier. The package further includes a lid adhered to the upper surface of the intermediate device for housing the circuit chip therein. The lid comprises a first, thermally conductive portion coupled to the electronic device for heat dissipation, and a second portion coupled to the device carrier between the device carrier and the first portion of the lid. The second portion comprises a material having a coefficient of thermal expansion substantially similar to or the same as the device carrier. Matching the thermal expansion characteristics of the lid with those of the device carrier minimizes the stress at the lid/device carrier interface when the package is heated. Consequently, the electrical and mechanical bonds at this interface remain substantially intact during, for example, electrical operations or when the package is heated for removal from the printed circuit board.
In a specific configuration, the second portion of the lid includes a perimetrical support member surrounding the circuit chip and the first portion of the lid comprises an overlying cover plate attached to the support member. The support member is preferably manufactured from the same, or essentially the same, material as the device carrier to maintain the bond therebetween during thermal expansion and contraction of the package device. The cover plate protects the support member. In addition, the cover plate comprises a thermally and electrically conductive portion coupled to the back side of the integrated circuit chip to provide conductive paths for the integrated circuit chip to, for example, overlying electrical elements and/or a heat sink.
In a preferred embodiment, the device carrier comprises a multi-layered ceramic substrate having an array of bump contacts for electrically coupling and mounting the chip to the substrate. In this embodiment, the support member is an annular ceramic ring attached to the ceramic substrate. Preferably, the ceramic ring is manufactured separately from the multi layered ceramic substrate and then laminated to the upper surface of the substrate after the individual layers have been co-fired. With this method of construction, the upper surface of the ceramic substrate within the ceramic ring remains substantially planar after the ring has been attached to the substrate. This facilitates the attachment of the integrated circuit chip to the bump contacts on the substrate.
The cover plate is preferably constructed of a material that has been matched to the thermal expansion characteristics of the support member so that the bond between the plate and the ceramic ring will remain substantially intact during thermal expansion and contraction of the package device. Suitable materials for the cover plate include tungsten, copper, molybdenum or their alloys. The ceramic ring preferably has a height substantially equal to the height of the integrated circuit chip above the ceramic substrate. With this configuration, the cover plate comprises a relatively flat plate attached to the upper surface of the ceramic ring slightly above the circuit chip. This geometry reduces the amount of thermally conductive (i.e., metallic) material required for manufacturing the lid, thereby reducing the overall cost of the package device.
In an exemplary embodiment, a conductive adhesive electrically and thermally couples the inner surface of the cover plate with the integrated circuit chip for connecting the chip to an exterior heat sink and to other overlying electrical elements. The adhesive effectively ensures that the cover plate will remain in contact with the circuit chip during transportation of the chip, thermal cycling, component removal and the like. Since the inner surface of the cover plate extends only slightly above the chip (due to the geometry of the ceramic ring), a minimum amount of conductive adhesive is required to couple the chip to the cover plate, which increases the thermal conductivity therebetween.
The present invention achieves these benefits in the context of known process technology. However, a further understanding of the nature of the advantages of the present invention may be realized by reference to the specification and attached drawings.